show: new extension for displaying various repository data

show: new extension for displaying various repository data

Currently, Mercurial has a number of commands to show information. And,
there are features coming down the pipe that will introduce more
commands for showing information.

Currently, when introducing a new class of data or a view that we
wish to expose to the user, the strategy is to introduce a new command
or overload an existing command, sometimes both. For example, there is
a desire to formalize the wip/smartlog/underway/mine functionality that
many have devised. There is also a desire to introduce a "topics"
concept. Others would like views of "the current stack." In the
current model, we'd need a new command for wip/smartlog/etc (that
behaves a lot like a pre-defined alias of `hg log`). For topics,
we'd likely overload `hg topic[s]` to both display and manipulate
topics.

Adding new commands for every pre-defined query doesn't scale well
and pollutes `hg help`. Overloading commands to perform read-only and
write operations is arguably an UX anti-pattern: while having all
functionality for a given concept in one command is nice, having a
single command doing multiple discrete operations is not. Furthermore,
a user may be surprised that a command they thought was read-only
actually changes something.

We discussed this at the Mercurial 4.0 Sprint in Paris and decided that
having a single command where we could hang pre-defined views of
various data would be a good idea. Having such a command would:

* Help prevent an explosion of new query-related commands
* Create a clear separation between read and write operations
  (mitigates footguns)
* Avoids overloading the meaning of commands that manipulate data
  (bookmark, tag, branch, etc) (while we can't take away the
  existing behavior for BC reasons, we now won't introduce this
  behavior on new commands)
* Allows users to discover informational views more easily by
  aggregating them in a single location
* Lowers the barrier to creating the new views (since the barrier
  to creating a top-level command is relatively high)

So, this commit introduces the `hg show` command via the "show"
extension. This command accepts a positional argument of the
"view" to show. New views can be registered with a decorator. To
prove it works, we implement the "bookmarks" view, which shows a
table of bookmarks and their associated nodes.

We introduce a new style to hold everything used by `hg show`.

For our initial bookmarks view, the output varies from `hg bookmarks`:

* Padding is performed in the template itself as opposed to Python
* Revision integers are not shown
* shortest() is used to display a 5 character node by default (as
  opposed to static 12 characters)

I chose to implement the "bookmarks" view first because it is simple
and shouldn't invite too much bikeshedding that detracts from the
evaluation of `hg show` itself. But there is an important point
to consider: we now have 2 ways to show a list of bookmarks. I'm not
a fan of introducing multiple ways to do very similar things. So it
might be worth discussing how we wish to tackle this issue for
bookmarks, tags, branches, MQ series, etc.

I also made the choice of explicitly declaring the default show
template not part of the standard BC guarantees. History has shown
that we make mistakes and poor choices with output formatting but
can't fix these mistakes later because random tools are parsing
output and we don't want to break these tools. Optimizing for human
consumption is one of my goals for `hg show`. So, by not covering
the formatting as part of BC, the barrier to future change is much
lower and humans benefit.

There are some improvements that can be made to formatting. For
example, we don't yet use label() in the templates. We obviously
want this for color. But I'm not sure if we should reuse the existing
log.* labels or invent new ones. I figure we can punt that to a
follow-up.

At the aforementioned Sprint, we discussed and discarded various
alternatives to `hg show`.

We considered making `hg log <view>` perform this behavior. The main
reason we can't do this is because a positional argument to `hg log`
can be a file path and if there is a conflict between a path name and
a view name, behavior is ambiguous. We could have introduced
`hg log --view` or similar, but we felt that required too much typing
(we don't want to require a command flag to show a view) and wasn't
very discoverable. Furthermore, `hg log` is optimized for showing
changelog data and there are things that `hg display` could display
that aren't changelog centric.

There were concerns about using "show" as the command name.

Some users already have a "show" alias that is similar to `hg export`.

There were also concerns that Git users adapted to `git show` would
be confused by `hg show`'s different behavior. The main difference
here is `git show` prints an `hg export` like view of the current
commit by default and `hg show` requires an argument. `git show`
can also display any Git object. `git show` does not support
displaying more complex views: just single objects. If we
implemented `hg show <hash>` or `hg show <identifier>`, `hg show`
would be a superset of `git show`. Although, I'm hesitant to do that
at this time because I view `hg show` as a higher-level querying
command and there are namespace collisions between valid identifiers
and registered views.

There is also a prefix collision with `hg showconfig`, which is an
alias of `hg config`.

We also considered `hg view`, but that is already used by the "hgk"
extension.

`hg display` was also proposed at one point. It has a prefix collision
with `hg diff`. General consensus was "show" or "view" are the best
verbs. And since "view" was taken, "show" was chosen.

There are a number of inline TODOs in this patch. Some of these
represent decisions yet to be made. Others represent features
requiring non-trivial complexity. Rather than bloat the patch or
invite additional bikeshedding, I figured I'd document future
enhancements via TODO so we can get a minimal implmentation landed.
Something is better than nothing.

--- a/contrib/wix/templates.wxs	Sun Apr 02 18:13:03 2017 -0700
+++ b/contrib/wix/templates.wxs	Fri Mar 24 19:19:00 2017 -0700
@@ -32,6 +32,7 @@
           <File Name="map-cmdline.changelog" KeyPath="yes" />
           <File Name="map-cmdline.compact" />
           <File Name="map-cmdline.default" />
+          <File Name="map-cmdline.show" />
           <File Name="map-cmdline.bisect" />
           <File Name="map-cmdline.xml" />
           <File Name="map-cmdline.status" />

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext/show.py	Fri Mar 24 19:19:00 2017 -0700
@@ -0,0 +1,137 @@
+# show.py - Extension implementing `hg show`
+#
+# Copyright 2017 Gregory Szorc <[e-mail]>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""unified command to show various repository information (EXPERIMENTAL)
+
+This extension provides the :hg:`show` command, which provides a central
+command for displaying commonly-accessed repository data and views of that
+data.
+"""
+
+from __future__ import absolute_import
+
+from mercurial.i18n import _
+from mercurial import (
+    cmdutil,
+    commands,
+    error,
+    registrar,
+)
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
+cmdtable = {}
+command = cmdutil.command(cmdtable)
+
+class showcmdfunc(registrar._funcregistrarbase):
+    """Register a function to be invoked for an `hg show <thing>`."""
+
+    # Used by _formatdoc().
+    _docformat = '%s -- %s'
+
+    def _extrasetup(self, name, func, fmtopic=None):
+        """Called with decorator arguments to register a show view.
+
+        ``name`` is the sub-command name.
+
+        ``func`` is the function being decorated.
+
+        ``fmtopic`` is the topic in the style that will be rendered for
+        this view.
+        """
+        func._fmtopic = fmtopic
+
+showview = showcmdfunc()
+
+@command('show', commands.formatteropts, _('VIEW'))
+def show(ui, repo, view=None, template=None):
+    """show various repository information
+
+    A requested view of repository data is displayed.
+
+    If no view is requested, the list of available views is shown and the
+    command aborts.
+
+    .. note::
+
+       There are no backwards compatibility guarantees for the output of this
+       command. Output may change in any future Mercurial release.
+
+       Consumers wanting stable command output should specify a template via
+       ``-T/--template``.
+
+    List of available views:
+
+    """
+    if ui.plain() and not template:
+        raise error.Abort(_('"hg show" cannot be used in plain mode because '
+                            'output is not stable'),
+                          hint=_('unset HGPLAIN and invoke with -T/--template '
+                                 'to control output'))
+
+    views = showview._table
+
+    if not view:
+        ui.pager('show')
+        # TODO consider using formatter here so available views can be
+        # rendered to custom format.
+        ui.write(_('available views:\n'))
+        ui.write('\n')
+
+        for name, func in sorted(views.items()):
+            ui.write(('%s\n') % func.__doc__)
+
+        ui.write('\n')
+        raise error.Abort(_('no view requested'),
+                          hint=_('use "hg show VIEW" to choose a view'))
+
+    # TODO use same logic as dispatch to perform prefix matching.
+    if view not in views:
+        raise error.Abort(_('unknown view: %s') % view,
+                          hint=_('run "hg show" to see available views'))
+
+    template = template or 'show'
+    fmtopic = 'show%s' % views[view]._fmtopic
+
+    ui.pager('show')
+    with ui.formatter(fmtopic, {'template': template}) as fm:
+        return views[view](ui, repo, fm)
+
+@showview('bookmarks', fmtopic='bookmarks')
+def showbookmarks(ui, repo, fm):
+    """bookmarks and their associated changeset"""
+    marks = repo._bookmarks
+    if not len(marks):
+        # TODO json output is corrupted; consider using formatter
+        ui.write(_('(no bookmarks set)\n'))
+        return
+
+    active = repo._activebookmark
+    longestname = max(len(b) for b in marks)
+    # TODO consider exposing longest shortest(node).
+
+    for bm, node in sorted(marks.items()):
+        fm.startitem()
+        fm.context(ctx=repo[node])
+        fm.write('bookmark', '%s', bm)
+        fm.write('node', fm.hexfunc(node), fm.hexfunc(node))
+        fm.data(active=bm == active,
+                longestbookmarklen=longestname)
+
+# Adjust the docstring of the show command so it shows all registered views.
+# This is a bit hacky because it runs at the end of module load. When moved
+# into core or when another extension wants to provide a view, we'll need
+# to do this more robustly.
+# TODO make this more robust.
+longest = max(map(len, showview._table.keys()))
+for key in sorted(showview._table.keys()):
+    cmdtable['show'][0].__doc__ += ' %s   %s\n' % (
+        key.ljust(longest), showview._table[key]._origdoc)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/templates/map-cmdline.show	Fri Mar 24 19:19:00 2017 -0700
@@ -0,0 +1,2 @@
+# TODO add label() once we figure out which namespace the labels belong on.
+showbookmarks = '{if(active, "*", " ")} {pad(bookmark, longestbookmarklen + 4)}{shortest(node, 5)}\n'

--- a/tests/test-command-template.t	Sun Apr 02 18:13:03 2017 -0700
+++ b/tests/test-command-template.t	Fri Mar 24 19:19:00 2017 -0700
@@ -1108,11 +1108,11 @@
 
   $ hg log --style notexist
   abort: style 'notexist' not found
-  (available styles: bisect, changelog, compact, default, phases, status, xml)
+  (available styles: bisect, changelog, compact, default, phases, show, status, xml)
   [255]
 
   $ hg log -T list
-  available styles: bisect, changelog, compact, default, phases, status, xml
+  available styles: bisect, changelog, compact, default, phases, show, status, xml
   abort: specify a template
   [255]
 

--- a/tests/test-log.t	Sun Apr 02 18:13:03 2017 -0700
+++ b/tests/test-log.t	Fri Mar 24 19:19:00 2017 -0700
@@ -148,7 +148,7 @@
 
   $ hg log -f -l1 --style something
   abort: style 'something' not found
-  (available styles: bisect, changelog, compact, default, phases, status, xml)
+  (available styles: bisect, changelog, compact, default, phases, show, status, xml)
   [255]
 
 -f, phases style

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-show.t	Fri Mar 24 19:19:00 2017 -0700
@@ -0,0 +1,117 @@
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > show =
+  > EOF
+
+No arguments shows available views
+
+  $ hg init empty
+  $ cd empty
+  $ hg show
+  available views:
+  
+  bookmarks -- bookmarks and their associated changeset
+  
+  abort: no view requested
+  (use "hg show VIEW" to choose a view)
+  [255]
+
+`hg help show` prints available views
+
+  $ hg help show
+  hg show VIEW
+  
+  show various repository information
+  
+      A requested view of repository data is displayed.
+  
+      If no view is requested, the list of available views is shown and the
+      command aborts.
+  
+      Note:
+         There are no backwards compatibility guarantees for the output of this
+         command. Output may change in any future Mercurial release.
+  
+         Consumers wanting stable command output should specify a template via
+         "-T/--template".
+  
+      List of available views:
+  
+       bookmarks   bookmarks and their associated changeset
+  
+  (use 'hg help -e show' to show help for the show extension)
+  
+  options:
+  
+  (some details hidden, use --verbose to show complete help)
+
+Unknown view prints error
+
+  $ hg show badview
+  abort: unknown view: badview
+  (run "hg show" to see available views)
+  [255]
+
+HGPLAIN results in abort
+
+  $ HGPLAIN=1 hg show bookmarks
+  abort: "hg show" cannot be used in plain mode because output is not stable
+  (unset HGPLAIN and invoke with -T/--template to control output)
+  [255]
+
+But not if a template is specified
+
+  $ HGPLAIN=1 hg show bookmarks -T '{bookmark}\n'
+  (no bookmarks set)
+
+  $ cd ..
+
+bookmarks view with no bookmarks prints empty message
+
+  $ hg init books
+  $ cd books
+  $ touch f0
+  $ hg -q commit -A -m initial
+
+  $ hg show bookmarks
+  (no bookmarks set)
+
+bookmarks view shows bookmarks in an aligned table
+
+  $ echo book1 > f0
+  $ hg commit -m 'commit for book1'
+  $ echo book2 > f0
+  $ hg commit -m 'commit for book2'
+
+  $ hg bookmark -r 1 book1
+  $ hg bookmark a-longer-bookmark
+
+  $ hg show bookmarks
+  * a-longer-bookmark    7b570
+    book1                b757f
+
+A custom bookmarks template works
+
+  $ hg show bookmarks -T '{node} {bookmark} {active}\n'
+  7b5709ab64cbc34da9b4367b64afff47f2c4ee83 a-longer-bookmark True
+  b757f780b8ffd71267c6ccb32e0882d9d32a8cc0 book1 False
+
+bookmarks JSON works
+
+  $ hg show bookmarks -T json
+  [
+   {
+    "active": true,
+    "bookmark": "a-longer-bookmark",
+    "longestbookmarklen": 17,
+    "node": "7b5709ab64cbc34da9b4367b64afff47f2c4ee83"
+   },
+   {
+    "active": false,
+    "bookmark": "book1",
+    "longestbookmarklen": 17,
+    "node": "b757f780b8ffd71267c6ccb32e0882d9d32a8cc0"
+   }
+  ]
+
+  $ cd ..